This invention relates to an apparatus for depositing silicon on a substrate. In another aspect, the invention relates to a method for depositing silicon.
The deposition of a thin film of silicon onto a substrate has commercial and industrial significance. One commercially important application of silicon deposition is in the manufacture of photovoltaic cells for the conversion of sunlight into electrical energy. Such photovoltaic cells are typically manufactured by depositing three silicon layers onto the substrate. These layers include a p-doped layer, an n-doped layer and an intrinsic (undoped) layer sandwiched between the two doped layers. In operation, light (photons) is absorbed by the intrinsic layer which creates electron-hole pairs in the intrinsic layer. An electric field is generated between the doped layers which causes the electrons and holes to migrate in opposite directions, thereby producing an electrical potential difference across the cell.
Each of the silicon layers mentioned above are typically deposited onto the substrate by the decomposition of silane (SiH.sub.4). Products of this decomposition include amorphous silicon and silicon hydride (SiH) which are allowed to deposit on the substrate in a suitable deposition chamber. Examples of presently used vapor deposition techniques for depositing silicon onto a substrate include chemical vapor deposition, ion deposition and radio frequency glow discharge deposition. In the radio frequency glow discharge method, a mixture of silane gas and inert gas such as argon is passed into an evacuated chamber containing the substrate. The silane is decomposed at an elevated temperature by a radio frequency plasma discharge. The doped layers are deposited by mixing in n-type dopants, such as hydrides of As, P and Sb, or p-type dopants, such as hydrides of B, Al and Ga, with the silane.
In all of the silicon vapor deposition techniques, various impurities may exist in the deposition chamber which can undesirably affect electronic or optical properties of the deposited silicon when such impurities deposit on the substrate in uncombined form or in compound with the silicon. For example, impurities in the intrinsic layer can change its band structure so as to reduce the magnitude of the electric field generated within the finished solar cell. Consequently, electron and hole migration between the doped layers is undesirably affected. Impurities in the intrinsic silicon layer of a solar cell can also act as recombination centers for the recombination of electrons and holes which were produced from photon absorption. Thus, as electron-hole pairs are lost, lower voltage and power output of the cell result.
With respect to specific impurities, metallic impurities in elemental form can be present in an RF glow discharge chamber. Such metallic impurities can be produced anywhere metal surfaces (i.e. chamber walls and electrodes) within the chamber contact the hot plasma. Plasma contact can cause chemical sputtering and surface vaporization that will allow atoms of the metal to be removed and introduced into the plasma. In addition, n and p-type dopants can be especially pernicious impurities in the deposition of an intrinsic silicon layer. Commercially available silane, which is introduced into the deposition chamber, typically contains dopants such as PH.sub.3 and AsH.sub.3 at the ppm level, and substantially lower levels of B.sub.2 H.sub.6.
One process directed toward reducing the level of impurities in deposited silicon involves the removal of dopant impurities such as AsH.sub.3, PH.sub.3 and B.sub.2 H.sub.6 from silane feedstock before it is introduced to the deposition chamber. According to this process, silane gas is irradiated with ultraviolet radiation of a wavelength in the range of 190 nm to 202 nm. The above-mentioned dopant impurities are thought to be dissociated by the radiation to form neutral products. The net result is the formation of solid, polymeric deposits on the wall of the reaction vessel.